Because at the core of every religion is a tiny grain of truth. It's 50% intentionally fabricated nonsense, 45% poetic license and the personal interpretation of someone long dead, and maybe 5% of it is truly profound and universal

If you look at religions, they have a lot of commonalities. There is an inhuman, unknowable creator, or source. Then there's some number of superhuman beings who serve or reject the creator, and may interact with humans. They don't interact with the creator directly though - they're also not human, but have some exaggerated human qualities

The abyss, the primal chaos, Ginnungagap for the Norse... It's the nothing that spawned and makes up everything. It's ever present, but you know it by acting in harmony with it, and destroy yourself by acting against it.

Then there's the pantheons, servants, great spirits, naga or what have you - they're assertions for how to live in harmony with it, some kind of greater being that is partially right and partially wrong. They're value systems, and they make mistakes in most myths, showing the flaws. Sometimes they created the world from the abyss/chaos, sometimes they created humans, sometimes they just stumbled upon us. Or sometimes aliens that created us as a slave race, depending on how you want to see it.

And then there's the reason for it - in one way or another, it's to become something greater through our time alive. Often to become strong or pure enough to be able to join the deities, or to be able to exist in the void without burning to nothing.

You do it through engaging in life - mindfully doing anything will teach you truths about the universe, and through various forms of introspective meditation (or prayer) to bring yourself more in harmony with your version of the truth.

That's all more like spirituality, but then you spend generations adding in some cautionary tales - we do live in a society after all, these are like bedtime stories that mix our history with the values our society prizes.

Often heroes grow spiritually they make distilled rules to guide the society to improve... Generally they're pretty reasonable (in the context of the original period)

But then sometimes a more temporary spiritual leader decides they don't like something - clearly it's unnatural and unaligned with the truth of existence because they really hate it... So obviously it's the will of the creator, and it gets tacked on to the guiding code.

And several hundred years later, once the religion has gained institutional power in a much larger and more hierarchical society, assholes just add in whatever is convenient. The core message is forgotten, there's endless stuff tacked on teaching morals or history that can be reinterpreted... Or maybe society just changed, and you have to drop some rules or lose the flock

Tldr: there's a core message of how to grow spiritually as a person, and a glimpse of something true about the nature of reality (in a very metaphorical, poetic kind of way). The promise of a reason and a goal speaks to everyone... But then they keep going, and bury the original message by teaching all sorts of other junk, often misinterpreted for an agenda centuries ago, in the same tone. Often misinterpreted today for an agenda.

(Side note, all ancient stories are super poetic and metaphorical, even historical ones... They're probably just more fun and more easily remembered when they're repeated around the fire for the next generation)